In an electrostatographic reproducing apparatus commonly used today, a photoconductive insulating member may be charged to a negative potential, thereafter exposed to a light image of an original document to be reproduced. The exposure discharges the photoconductive insulating surface in exposed or background areas and creates an electrostatic latent image on the member which corresponds to the image areas contained within the original document. Subsequently, the electrostatic latent image on the photoconductive insulating surface is made visible by developing the image with a developing powder referred to in the art as toner. During development, the toner particles are attracted from the carrier particles by the charge pattern of the image areas on the photoconductive insulating area to form a powder image on the photoconductive insulating area. This image may be subsequently transferred or marked onto a support surface such as copy paper to which it may be permanently affixed by heating and/or by the application of pressure. Following transfer of the toner image or marking, the copy paper may be removed from the system by a user or may be automatically forwarded to a finishing station where the copies may be collected, compiled and stapled and formed into books, pamphlets or other sets.
Image consistency is important whether the copies are collected or compiled and formed into books, pamphlets, etc. One important property of print quality is the uniformity of the print. Many parameters of the xerographic process affect print uniformity, but one of the most important ones is charge uniformity since that is where the process starts. Measurements taken on drums with electrostatic volt meters show that the charge uniformity on the drum is periodic with the drum and very predictable. These measurements also show that the PR voltage patterns are very similar in shape and amplitude for all halftone prints and the solid print, i.e. from zero to full discharge. The present invention proposes to measure the PR voltage pattern during a number of drum (or belt) revolutions at the charge and latent imaging levels, store the observed voltage pattern and correct it by adjusting the charge device so that it is essentially constant. The measurement of the charge and discharge patterns can be repeated as needed to accommodate changes in the pattern over time.
There have been some attempts to control the charging step of the electrostatic process such as that disclosed by U.S. Pat. No. 4,417,804 (Werner). The Werner invention is concerned with a photoreceptor voltage control comprising a comparator circuit for determining the error between the photoreceptor voltage and the desired voltage. The photoreceptor voltage is detected by a non-contacting detector and the photoreceptor voltage signal is fed directly to the comparator circuit which determines if the error is too positive, too negative or within acceptable limits. This information is then fed by a DC isolation system to the machine logic control which in turn corrects a corona supply voltage to obtain the desired photoreceptor voltage. In Werner, his voltage detector 18 is placed immediately after the charging station 12, thereby establishing a desirable voltage of the photoreceptor only after this charging step. One disadvantage of the Werner control system is that the voltage detector is fully dedicated to the charge system and its control loop and the correction performed is reactive and always a little delayed because of the sensor position after the charge device. This invention is proposing a predictive control method by measuring the PR voltage pattern before printing starts, store the pattern, and adjust the charge output exactly as needed. Another advantage of this system over the Werner system is that the Werner system looks solely at the charge voltage, whereas this invention can also measure exposed areas of the photoreceptor which can be more optimal for achieving uniform print quality.